1. Field of the Invention
The present invention relates to a novel complex of an electron attracting group-substituted .eta..sup.5 -cyclopentadienylcobalt with a polyene or an acetylene, and a process for preparing a pyridine homologue from an alkyne and a nitrile by using the complex as a catalyst.
2. Description of the Prior Art
Cyclopentadienylcobalt cyclic diene complexes and their use as catalysts for the synthesis of a substituted pyridine from an alkyne and a nitrile, have already been known. For instance, cycloalkadienylcobalt cycloalkadiene complexes and their use as catalysts for the cyclization of two molecules of an alkyne and one molecule of an nitrile at the respective triple bond portions to form a substituted pyridine, have been known (Japanese Unexamined Patent Publications No. 135084/1975 and No. 25780/1977 and U.S. Pat. No. 4,267,329). However, these catalysts do not provide an adequate catalytic activity.
A synthesis of pyridine or its homologues by reacting an alkyne with a nitrile to co-cyclize them at their triple bonds a single step is a superb process. Some of the present inventors have previously disclosed that a cobalt compound, particularly an .eta..sup.5 -cyclopentadienylcobalt derivative, is an effective catalyst for such a reaction (Japanese Examined Patent Publications No. 13153/1976 and No. 15597/1977). However, a catalyst having a still higher catalytic activity has been desired.
As a highly active catalyst for such a reaction, it has been proposed to use a borabenzene cobalt complex (European Pat. No. 64268). However, this catalyst has drawbacks that its preparation is cumbersome and it is very expensive.
It is accordingly an object of the present invention to provide a highly active catalyst which can readily be prepared and is useful for the synthesis of a pyridine homologue from an alkyne and a nitrile.
As a result of extensive researches, the present inventors have found that a cobalt complex having an electron attractive substituent on its cyclopentadienyl ring exhibits a superior catalytic activity. The present invention has been accomplished based on this discovery.
Thus, the present invention provides, as a novel compound, a complex of an electron attracting group-substituted .eta..sup.5 -cyclopentadienylcobalt with a polyene or an acetylene, and a process for preparing a pyridine homologue which comprises reacting an alkyne with a nitrile in the presence of the novel compound as the catalyst.
The novel complex of the present invention is represented by the general formula: ##STR1## where A is a C.sub.1 -C.sub.4 alkyl group and/or a phenyl group, n is an integer of from 0 to 2, B is an alkoxycarbonyl group in which the alkoxy moiety contains from 1 to 3 carbon atoms or a C.sub.1 -C.sub.4 acyl group, m is an integer of 1 or 2, and Q is --Co.dbd.R.sub.1 (where R.sub.1 is a C.sub.4 -C.sub.12 polyene having from 2 to 4 double bonds, which is unsubstituted or substituted by from 1 to 4 substituents selected from the group consisting of a C.sub.1 -C.sub.4 alkyl group, a C.sub.1 -C.sub.4 alkoxycarbonyl group, a phenyl group, a cyano group and a cyanomethylene group and which forms a coordinate bond with the cobalt atom at its diene portion, provided R.sub.1 excludes a polyene composed solely of an aromatic ring), ##STR2## (where R.sub.2 is an organic residue of a metallo-cyclic cyclopentadiene which is unsubstituted or substituted by from 1 to 4 substituents selected from the group consisting of a C.sub.1 -C.sub.4 alkyl group, a C.sub.1 -C.sub.4 alkoxycarbonyl group, a phenyl group and a cyano group), ##STR3## (where R.sub.3 is an acetylene which is substituted by one or two substituents selected from the group consisting of a C.sub.1 -C.sub.4 alkyl group, a C.sub.1 -C.sub.4 alkoxycarbonyl group, a phenyl group and a cyano group and which forms a coordinate bond with the cobalt atom at its triple bond portion), or ##STR4## (where R.sub.2, A, B, n and m are as defined above).
It used to be difficult to prepare a substituted .eta..sup.5 -pentadienyl cobalt complex having an electron attracting substituent on its cyclopendadienyl ring. The present inventors have solved the difficulty by developing the following novel methods for its preparation.
(i) A method in which an alkoxycarbonyl- or acyl-substituted cyclopentadienyl alkali metal complex is reacted with a tris-triphenylphosphine cobalt monohalogenide, followed by a reaction with a polyene or an acetylene to obtain the desired complex. PA1 (ii) A method in which an alkoxycarbonyl- or acyl-substituted cyclopentadienyl alkali metal complex, a cobalt(II) halide and a polyene or an acetylene, are reacted in the presence of a reducing agent.
In the process for the synthesis of pyridine or its homologue by reacting an alkyne with a nitrile, the complex of an electron attracting group-substituted cyclopentadienylcobalt with a polyene of an acetylene exhibits an extremely high catalytic activity. Expecially, the complex having an alkoxycarbonyl group or an acyl group as the electron attractive substituents, is particularly effective to provide a far superior reaction rate as compared with conventional catalysts of this type, and its catalytic efficiency is as high as from 1.5 to 20 times that of the conventional catalysts. Further, it effectively serves to suppress the formation of aromatic hydrocarbons as by-products of the reaction to a level of at most 1/10. Furthermore, by virtue of the high catalytic activity of the complex, it is possible to substantially reduce the pressure for the reaction. This is extremely effective to prevent decomposition or explosion when a lower acetylene is used as the starting material. Thus, this is very important for an industrial application.
The general formula I covers the following types of complexes: ##STR5## In the formulas II to V, A, B, n, m, R.sub.1, R.sub.2 and R.sub.3 are as defined above with respect to the general formula I. The complex of the formula III is a substance in which two molecules of acetylene or its derivative (R.sub.2 ') are reacted, and the triple bond portion of the acetylene in the reaction product forms together with cobalt a metallocyclic cyclopentadiene ring, and triphenylphosphine forms a coordinate bond with cobalt. The complex of the formula V has a structure in which instead of the triphenylphosphine in the formula III, a substituted .eta..sup.5 -cyclopentadienylcobalt is bonded to form a cobalt-cobalt bond, which further establishes a diene coordination with the cobaltacyclopentadiene ring.
The polyene to be used in the present invention may be selected from a wide range of polyenes. For instance, there may be mentioned butadiene, isoprene, cyclopentadiene, dicyclopentadiene, hexadiene, norbornadiene, indene, cyclooctadiene, cyclooctatetraene, azulene, cyclododecatriene, or alkyl- or phenyl-substituted derivatives thereof or alkoxycarbonyl-, cyano- or cyanomethylene substituted derivatives thereof. However, polyenes composed solely of an aromatic ring such as benzene, toluene or xylene are not useful for the present invention as they are not reactive.
As the acetylene to be used in the present invention, there may be mentioned acetylene, methylacetylene, ethylacetylene, hexadiyne, phenylacetylene, or alkoxycarbonyl- or cyano-substituted derivatives thereof.
The complexes of the present invention are all novel compounds, and their structures have been ascertained by elementary analysis, infrared spectrum, and NMR.
The unsubstituted or substituted .eta..sup.5 -alkoxycarbonyl- or .eta..sup.5 -acyl-cyclopentadienylcobalt complexes of the present invention have been found to exhibit an extremely high catalytic activity when used as catalysts for the synthesis of an unsubstituted or substituted pyridine from an alkyne and a nitrile. Cycloalkadienylcobalt complexes usually have a catalytic activity for the synthesis of pyridine derivatives by co-cyclization reaction of an alkyne and a nitrile. However, cyclopentadienylcobalt complexes having an electron attracting substituent on its cyclopentadienyl ring have a higher catalytic activity.
The unsubstituted or substituted .eta..sup.5 -alkoxycarbonyl- or .eta..sup.5 -acyl-cyclopentadienylcobalt complexes of the present invention exhibit especially good catalytic activity.
When the catalyst is used in an amount of at least 0.1 mmol/liter in the reaction system, an effective catalytic activity is obtainable. It is usually unnecessary to bring the concentration higher than 100 mmol/liter.
The starting materials may be selected from wide ranges of alkynes and nitriles when the catalyts of the present invention are used, like in the cases where conventional catalysts are used.
As the alkynes, there may be employed acetylene, a mono substituted alkyne such as an alkyl-, alkenyl- or aryl-acetylene, a di-substituted acetylene such as a dialkyl-, diaryl-, alkyl-alkenyl- or alkyl-aryl-acetylene, or a mixture of such alkynes. Further, an ether- or alcohol- derivative thereof may also be used.
On the other hand, as the nitriles, there may be advantageously employed a mononitrile such as hydrogen cyanide or an alkyl-, aryl- or alkenyl-nitrile, or a polyfunctional nitrile having a plurality of nitrile groups.
The molar ratio of the alkyne to the nitrile may be optionally selected within a range of from 0.01 to 100. However, in order to minimize the formation of by-products, it is preferred that the nitrile is used in excess of the amount of the alkyne.
The process of the present invention may be conducted in such a manner that the catalyst is added to a nitrile in the presence or absence of a solvent, and an alkyne is added at a temperature of from 15.degree. to 200.degree. C., whereby the reaction will proceed. As the solvent, there may be used a variety of solvents such as an aromatic hydrocarbon, an alcohol, an amine, an ether, an ester or an alkylcarboxyamide. However, such a solvent is not necessarily required since the starting material nitrile serves as a solvent.
The complex obtained according to the present invention may directly be used for the synthesis of a pyridine homologue without isolation or purification before use. This is extremely advantageous for the practical application of the catalyst to an industrial operation.
The feature of the process of the present invention is that the catalytic activity and the reaction rate are high.
Another important feature is that it is thereby possible to substantially reduce aromatic hydrocarbons which are likely to form as by-products in the synthesis of the pyridine homologues. Accordingly, it is readily possible to attain the productivity and the concentration of the product which adequately satisfy the industrial requirements.
Especially when a lower alkyne such as acetylene is used as a starting material, it is possible to conduct the reaction at a pressure substantially lower than that of the conventional reaction system, i.e. under pressure of from 0 to 30 kg/cm.sup.2 G, preferably from 5 to 20 kg/cm.sup.G and at an adequately practical reaction rate. This is practically very important in that the lower alkyne is likely to lead to decomposition or explosion when pressurized.
It may be said that it has been made possible for the first time by the present invention to provide readily and in an industrial scale an industrially important product such as vinyl pyridine, .alpha.-picoline or pyridine from acetylene and a nitrile.
Heretofore, an .eta..sup.5 -cyclopentadienylcobalt polyene complex has been usually prepared by reacting dicarbonyl-.eta..sup.5 -cyclopentadienylcobalt with a polyene or an acetylene [Shin Jikken Kagaku Koza Vol. 12, pages 189-190 (Maruzen)]. However, according to such a conventional process, it has difficult to produce an electron attracting group-substituted .eta..sup.5 -cyclopentadienylcobalt polyene complex and an electron attracting group-substituted .eta..sup.5 -cyclopentadienylcobalt monoyne complex of the present invention. As a result of extensive research, the present inventors have found novel methods for the synthesis. The present invention has been accomplished by this discovery.
The .eta..sup.5 -alkoxycarbonyl- or .eta..sup.5 -acyl-cyclopentadienylcobalt complex of the present invention is prepared by the following routes. ##STR6##
In the formula VI, M is an alkali metal and X is a halogen atom, and in the formula VII, R.sub.1 is a C.sub.4 -C.sub.12 polyene having from 2 to 4 double bonds, which is unsubstituted or substituted by from 1 to 4 substituents selected from the group consisting of a C.sub.1 -C.sub.4 alkyl group, a C.sub.1 -C.sub.4 alkoxycarbonyl group, a phenyl group, a cyano group and a cyanomethylene group. However, R.sub.1 excludes a polyene composed solely of an aromatic ring. A is a hydrogen or a C.sub.1 -C.sub.4 alkyl group and/or a phenyl group, n is 0 to 2, B is an alkoxycarbonyl group in which the alkoxy moiety contains from 1 to 4 carbon atoms, or an acyl group, and m is 1 or 2.
Instead of the process of the formula VII, a process of the formula VIII may be employed. ##STR7##
In the formula VIII, R.sub.2 ' is an acetylene which is unsubstituted or substituted by one or two substituents selected from the group consisting of a C.sub.1 -C.sub.4 alkyl group, a C.sub.1 -C.sub.4 alkoxycarbonyl group, a phenyl group and a cyano group, and R.sub.2 is an organic residue of a metallocycliccyclopentadiene, which is composed of 2 molecules or R.sub.2 ' and a cobalt atom, which may have the same substituents as R.sub.2 '.
Instead of the process of the formula VII, a process of the formula IX may be employed. ##STR8##
In the formula IX, R.sub.3 is a substituted acetylene having the same substituents as R.sub.2 '.
Instead of the process of the formula VII, a process of the formula X may be employed, ##STR9##
In the formula X, R.sub.2 ' and R.sub.2 are as defined above with respect to the formula VIII. Instead of the triphenylphosphine in the formula VIII, a substituted .eta..sup.5 -cyclopentadienylcobalt is bonded to form a cobalt-cobalt bond, which further establishes a diene coordination with the cobaltacyclopentadiene ring.
The starting material of the formula VI such as an alkoxycarbonyl- or acyl-cyclopentadienyl sodium may be obtained by the process of Rausch [J. Am. Chem. Soc., 102, 1196 (1980)].
The alkoxycarbonyl group or the acyl group represented by B includes a methoxycarbonyl group, an ethoxycarbonyl group, an isopropoxycarbonyl group, a formyl group, an acetyl group, a propionyl group or a butylyl group, and it is synthesized respectively from the corresponding carbonate, chloroformate or carboxylate and a cyclopentadienyl alkali metal complex.
The introduction of an alkyl group and/or a phenyl group may be conducted by a conventional method, for instance, by reacting an alkali metal complex such as a cyclopentadienyl sodium with a corresponding halogenated alkyl or phenyl, and, if necessary, repeating this reaction. This reaction is preferably conducted prior to the introduction of the B group.
The tris-triphenylphosphine cobalt monohalogenides may readily be obtained by reacting a cobalt halide with triphenylphosphine in the presence of a reducing agent [Inorg. Chim. Acta., 3, 227 (1969)].
The reaction of the formula VI may be conducted by reacting a substantially equimolar amount or a slight excess thereof of an alkoxycarbonyl- or acyl-cyclopentadienyl alkali metal complex in a solvent at a temperature of from 0.degree. to 80.degree. C. for 0.1 to 24 hours. The reaction product thereby obtained may be isolated and purified, but may directly be used for the subsequent reaction without such isolation or purification. The reactions of the formulas VII, VIII, IX and X may be conducted by adding a polyene or an acetylene corresponding to the desired substance to the reaction solution of the formula VI and reacting the mixture at a temperature of from 0.degree. to 150.degree. C. for from 0.5 to 24 hours.
These reactions may preferably be conducted in an inert atmosphere free from the presence of oxygen. After concentrating the reaction product solutions, the reaction products may be isolated or purified by column chromatography.
The substituted .eta..sup.5 -cyclopentadienylcobalt polyene complexes thus obtained are stable in air.
A feature of the present invention resides in that a cyclopentadienyl alkali metal complex wherein one or two hydrogen atoms on the ring are substituted by an alkoxycarbonyl group or an acyl group, is used as a starting material, and the starting material is reacted with a tris-triphenylphosphine cobalt monohalogenide and the reaction product solution is directly reacted with a corresponding polyene or acetylene. The reaction of the formula VI has been known wherein a cyclopentadienyl sodium is used as a starting material. However, an alkoxycarbonyl- or acyl-cyclopentadienyl alkali metal complex has been used as the starting material for the first time by the present invention. Furthermore, the reaction of this reaction product with a polyene or an actylene has not been known before.
The .eta..sup.5 -alkoxycarbonyl-, and .eta..sup.5 -acyl-cyclopentadienylcobalt complexes represented by the general formula II may also be prepared by the following method.
Namely, (i) at least one of substituted cyclopentadienylcobalt monohalogenides represented by the formula XI: ##STR10## where A, B, X, m and n are as defined above, with respect to the formula XI, and (ii) at least one of C.sub.1 -C.sub.16 polyenes having from 2 to 4 double bonds, excluding a polyene composed solely of an aromatic ring, are mixed, and the mixture is reacted with (iii) a reducing agent, whereby an electron attracting group-substituted .eta..sup.5 -cyclopentadienylcobalt-.eta..sup.4 -polyene complex of the formula II is prepared.
Alternatively, (i) at least one of substituted cyclopentadienyl alkali metal complexes of the formula VI, and (ii) at least one of cobalt(II) halide, are mixed and reacted to form a corresponding .eta..sup.5 -substituted cyclopentadienylcobalt monohalogenide, and without isolation or separation, this reaction product is directly reacted with (iii) at least one of C.sub.4 -C.sub.16 polyenes having from 2 to 4 double bonds, excluding a polyene composed solely of an aromatic ring, in the presence of (iv) a reducing agent to obtain an electron attracting group-substituted .eta..sup.5 -cyclopentadienylcobalt-.eta..sup.4 -polyene complex of the formula II. In this process, the polyenes (iii) may be present at the time of the reaction of the reactants (i) and (ii).
Cyclopentadienylcobalt monohalogenides are known compounds. Whereas, the substituted cyclopentadienylcobalt monohalogenides of the present invention are all novel compounds. The substituted cyclopentadienylcobalt monohalogenide may be prepared by reacting a substituted cyclopentadienyl alkali metal complex of the formula VI with a cobalt(II) halide. It has been found that in this case, it is important to react the two reactants in a ratio close to equimolar amounts as far as possible. If the amount of the cobalt salt is too small, by-products of cobaltcene type tend to increase. On the other hand, if the amount is excessive, unreacted substances tend to increase. The reaction may readily be carried out in the presence of a solvent at a temperature of from -20.degree. to 50.degree. C. As the solvent, an inert solvent such as a hydrocarbon or an ether may preferably be used. However, the above-mentioned polyene may be used as a solvent. From the reaction product solution thus obtained, the substituted cyclopentadienylcobalt mono-halogenide may be isolated. However, the reaction solution may be directly be used for the subsequent reaction.
It has been found that the substituted cyclopentadienylcobalt monohalogenide and the polyene can readily be converted by the action of a reducing agent to a substituted cyclopentadienylcobalt polyene complex. Heretofore, it has been known to obtain a catalyst active for the synthesis of a pyridine derivative by reacting a cobalt salt with an alkyne or a nitrile in the presence of a reducing agent. However, it has not been known to use a substituted cyclopentadienylcobalt monohalogenide as the starting material, as in the present invention. The process for preparing a substituted cyclopentadienylcobalt polyene complex readily and in good yield has been found for the first time by the present invention.
The molar ratio of the substituted cyclopentadienylcobalt monohalogenide and the polyene is within a range of from 1 to 200. As the reducing agent, there may be used an alkali metal amalgam, hydride, borohydride, aluminum hydride, an alkyl aluminum or an alkyl zinc. The amount of the reducing agent is preferably at least equivalent to the amount of cobalt. The reaction is conducted at a temperature within a range of from -20.degree. to 100.degree. C. in the presence or absence of a solvent.
As the solvent, a hydrocarbon or an ether may preferably be used. The polyene as a reactant or the solution system of the preceeding step may be employed for this reaction. The reaction is preferably conducted in an inert atmosphere containing no oxygen, moisture or acidic gas under atmospheric pressure or elevated pressure. The reaction time may vary depending upon the temperature and the solvent, but is usually within 1 hour.
From the reaction product solution thus obtained, a pure product of the substituted .eta..sup.5 -cyclopentadienylcobalt-.eta..sup.4 -polyene complex may be obtained. This isolation operation can more readily be conducted than the isolation operation of the above-mentioned system wherein triphenylphosphine is used. However, when the complex is used as a catalyst for the process of the present invention, such an isolation operation is not necessarily required, and the reaction product solution may be used as it is. This system is particularly advantageous as compared with the above-mentioned system wherein is used triphenylphosphine, since the expensive isolation operation is not required.